A quantum-confined Stark effect semiconductor optical modulator, operable to modulate light of a particular wavelength in the range of around 780 to 840 nm. A p-i-n diode having p, intrinsic and n regions, as well as first and second electrical contacts for application of a reverse bias voltage defines the modulator. The particular intrinsic region includes a plurality of semiconductor layers defining a plurality of quantum wells separated by barrier layers having a certain bandgap energy above that of the quantum wells. The quantum wells including at least two ultra-thin barrier layers within the quantum well and being of a material having a certain bandgap energy above that of the quantum wells. The width of each ultra-thin barrier layer is no more than approximately two molecular layers thick.